Ink jet printing is a non-impact and non-contact printing process in which an electronic signal produces droplets of ink that are deposited on a wide variety of substrates such as paper, transparent film, plastics, metals and fabrics. It is extremely versatile in substrate variety, in print quality and in speed of operation. It is digitally controllable. For these reasons, ink jet is widely used industrially for marking and labeling. In the office it has been broadly utilized as output for personal computers. This is especially true for thermal or bubble jet drop-on-demand printers with disposable ink cartridges. These printers have been widely received due to their high print quality, low cost, relatively quiet operation and graphics capability.
Both dyes and pigments have been used as ink colorants for ink jet printers. However, dyes have several disadvantages. They are water-soluble and remain so after drying. They are redissolved by contact with water and will run when exposed to a water spill. Also dye images smear on contact with felt pen markers. In addition they exhibit poor light stability relative to pigments and are known to fade even under conditions of office fluorescent lighting. Thus, there are several disadvantages with the use of dye-based ink jet inks, many of which prohibit their use in applications requiring fast drying times and greater light stability.
Pigment colorants are being increasingly used due to their improved water-and smear resistance, and improved lightfastness, compared to dyes. The inclusions of cosolvents and penetrants to pigment-based inks, however, tends to destabilize pigment dispersions. Thus, pigments are a useful alternative to dye colorants provided the dispersions can be made stable to flocculation and settling.
Three competing factors dominate the formulation of ink for ink jet printers: (1) drying rate, (2) print quality, and (3) reliability. Drying rate determines the throughput rate and therefore productivity of a printer. One major deficiency of thermal ink jet printers is their relatively slow printing speed compared to printers using competing technologies. A major obstacle is the ink drying rate. In a sheet fed printer, the ink must be dry before the succeeding sheet contacts it or the ink will smear.
Drying occurs by both evaporation and penetration. Evaporation is determined by solvent vapor pressure whereas penetration is determined by interfacial energy between the ink and the paper and the porosity of the print media. The use of penetrants is known in the art as a means to increase the rate of penetration of inks. Conventional penetrants such as Butyl Carbitol.RTM., however, tend to cause pigment dispersions to flocculate and have limited use in pigmented ink jet inks.
Print quality can be somewhat subjective in measurement. Basically it is the image definition or contrast verses the nonimage background areas. It is controlled by 2 general factors:
1) Color properties of the ink measured by optical density and color coordinates controlling hue, e.g. L*, A* and B* in the CIELAB 1976 color standards. In the case of black inks the optical density is the most important of these two factors.
2) Non-color image characteristics that determine image definition are: a) resolution, i.e. number of drops of ink per unit area, b) the area coverage per drop, c) edge acuity, and d) presence or absence of artifacts such as satellite droplets, side pennants or feathering. The latter is a critical phenomena because it is often the result of a fast penetrating ink which conducts ink through paper fibers, producing a fuzzy or feathery image. The cosolvents of this invention are able to penetrate rapidly without producing feathering.
An important reliability issue is decap or crust time which is defined as the time span over which a pen can be exposed to the atmosphere without failing to print. Other issues are stability of the ink caused by physical or chemical changes, compatibility with equipment material, robustness towards variability in thermal firing conditions and stable drop volume over long term use.
These three factors often compete with one another. For example, methods for decreasing drying rate tend to adversely affect either print quality or reliability. Selecting a more volatile cosolvent will decrease drying time, but also tends to decrease decap time. Selection of a more penetrating solvent will lower drying rate, but will also lower optical density and induce feathering. Increasing optical density by employing more colorant can adversely effect reliability. Cosolvents are available that increase penetration rate, but many of them will destabilize pigment dispersions.
Accordingly, a need exists for penetrants that will increase ink drying without degrading print quality through feathering and without destabilizing pigment-based inks.